There is known a surface acoustic wave (SAW) device as an acoustic wave device using an acoustic wave. The SAW device is compact and light and is capable of attenuating signals greatly. Therefore, the SAW device is used as a filter for mobile communications such as portable phones.
Recently, a Lamb wave device and a film bulk acoustic resonator (FBAR) that use a thin piezoelectric layer have been proposed as devices obtained by developing the SAW device (see Japanese Patent Application Publication No. 2010-136317: Document 1). An acoustic wave device using a thin piezoelectric layer may have a structure in which a support substrate having a recess portion is bonded to a piezoelectric layer, and an electrode is provided to an upper surface of the piezoelectric layer or both the upper and lower surfaces thereof above a hollow portion defined by the recess portion.
There are various proposed methods for manufacturing the acoustic wave device utilizing the thin piezoelectric layer. For example, Document 1 discloses that the recess portion is formed in an area of the support substrate that is to be formed into the hollow portion, and is filled with a sacrificing layer beforehand. A piezoelectric substrate is bonded to the support substrate, and is then thinned to form the piezoelectric layer. Then, the electrode is formed on the upper surface of the piezoelectric layer. Finally, the sacrificing layer is removed through a via hole that is formed beforehand, whereby the hollow portion can be defined.
The method proposed in Document 1 is capable of preventing an unevenness of the thickness of the piezoelectric layer, as compared with a comparative case where the piezoelectric substrate is bonded to the support substrate having the recess portion that is not filled with the sacrificing layer (see paragraph 0142 and FIG. 36 of Document 1). However, there is a room left for improvement in the manufacturing method when aspects of productivity and easy manufacturability are taken into consideration.